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Urinary Osmolality Gap
Estimates urinary ammonium secretion and unmeasured osmoles
Laboratory Inputs
Required (50–1400)
Required (0–400)
Required (0–300)
Required (0–3000)
Required (0–5000)
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Calc. Uosm (mOsm/kg)
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Osmolal Gap (mOsm/kg)
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Est. NH₄⁺ (mEq/L)
| Gap | Interpretation | Clinical Significance |
|---|---|---|
| < 10–15 | Normal | Low NH₄⁺ excretion or no unmeasured osmoles |
| 40–200 | Moderate | Moderate NH₄⁺ excretion; good renal acidification |
| > 400 | High | Excellent renal NH₄⁺ excretion — appropriate response to acidosis |
| Very high | Unmeasured osmoles | Consider toxic alcohols (methanol, ethylene glycol) |
Calculated Uosm = 2×(UNa + UK) + UUrea/2.8 + UGlucose/18
Osmolal Gap = Measured Uosm − Calculated Uosm
Est. NH₄⁺ ≈ Osmolal Gap / 2
Normal gap: <10–15 mOsm/kg
Osmolal Gap = Measured Uosm − Calculated Uosm
Est. NH₄⁺ ≈ Osmolal Gap / 2
Normal gap: <10–15 mOsm/kg
Clinical Use in Acidosis: Urinary osmolal gap correlates with NH₄⁺ excretion. Gap >400 mOsm/kg indicates robust renal acid excretion (appropriate response). Low gap in acidosis suggests impaired NH₄⁺ excretion (distal RTA or type IV RTA). Complements the urine anion gap when NH₄Cl is excreted without Cl (e.g., with ketones or other anions present).
